Method and apparatus for video coding

ABSTRACT

Aspects of the disclosure provide methods and apparatuses for video encoding/decoding. In some examples, an apparatus for video decoding includes receiving circuitry and processing circuitry. The processing circuitry decodes prediction information of a current block in a current picture from a coded video bitstream. The prediction information is indicative of an affine based motion vector prediction. The processing circuitry determines a first affine model for the current block. The first affine model has first affine parameters. Then, the processing circuitry determines a second affine model for a first clipping region in the current block. The second affine model has second affine parameters with differences to the first affine parameters being smaller than a threshold. Motion vectors determined based on the second affine model satisfy a clipping requirement. The processing circuitry reconstructs a sample in the first clipping region based on the second affine model.

INCORPORATION BY REFERENCE

This present application claims the benefit of priority to U.S.Provisional Application No. 62/791,793, “Sub-block Motion VectorClipping” filed on Jan. 12, 2019, which is incorporated by referenceherein in its entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs ofneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described here is atechnique henceforth referred to as “spatial merge”.

Referring to FIG. 1, a current block (101) comprises samples that havebeen found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (102 through 106, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videoencoding/decoding. In some examples, an apparatus for video decodingincludes receiving circuitry and processing circuitry. The processingcircuitry decodes prediction information of a current block in a currentpicture from a coded video bitstream. The prediction information isindicative of an affine based motion vector prediction. The processingcircuitry determines a first affine model for the current block. Thefirst affine model has first affine parameters. Then, the processingcircuitry determines a second affine model for a first clipping regionin the current block. The second affine model has second affineparameters with differences to the first affine parameters being smallerthan a threshold. Motion vectors determined based on the second affinemodel satisfy a clipping requirement. The processing circuitryreconstructs a sample in the first clipping region based on the secondaffine model.

In some embodiments, the processing circuitry determines first motionvectors for sub-blocks in the first clipping region based on the firstaffine model, and then clips the first motion vectors to generate secondmotion vectors that satisfy the clipping requirement. Further, theprocessing circuitry determines the second affine model based on thesecond motion vectors of the sub-blocks.

In some examples, the processing circuitry reconstructs a sample in asecond clipping region within the current block based on the secondaffine model.

In some embodiments, the processing circuitry determines first motionvectors for sub-blocks in each clipping region of the current blockbased on the first affine model, and clips the first motion vectors foreach clipping region of the current block to generate second motionvectors for respective clipping regions that satisfy the clippingrequirement. Further, the processing circuitry determines respectivesecond affine models for the clipping regions based on the second motionvectors for the respective clipping regions, and constrains parametersof the second affine models with differences to the first affineparameters being smaller than the threshold. Then, the processingcircuitry reconstructs samples in the clipping regions based onrespective second affine models for the clipping regions.

In some examples, the processing circuitry clips the first affineparameters based on a range requirement to generate the second affineparameters for the second affine model. In an example, the processingcircuitry determines motion vectors of sub-blocks for each clippingregion based on the second affine model, and clips the motion vectors ofthe sub-blocks. In another example, the range requirement is predefined.In another example, the processing circuitry decodes a signal from thecoded video bitstream that is indicative of the range requirement. Insome examples, the current block is a bi-predicted block.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform the method forvideo decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1 is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example.

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system (200) in accordance with an embodiment.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 8 is a schematic illustration of spatial neighboring blocks inaccordance with an embodiment.

FIG. 9 shows a diagram for illustrating an affine model.

FIG. 10 shows a diagram illustrating motion vector clipping in a firstclipping region.

FIG. 11 shows a diagram illustrating the first clipping region.

FIG. 12 shows a flow chart outlining a process example according to someembodiments of the disclosure.

FIG. 13 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230) and(240) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230) and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (313), that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3 can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data (304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (420) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (415), so as tocreate symbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values, thatcan be input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (452) has generated to the outputsample information as provided by the scaler/inverse transform unit(451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (453) in the form of symbols (421) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (457) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols (421) from the parser (420), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder (503) isincluded in an electronic device (520). The electronic device (520)includes a transmitter (540) (e.g., transmitting circuitry). The videoencoder (503) can be used in the place of the video encoder (303) in theFIG. 3 example.

The video encoder (503) may receive video samples from a video source(501) (that is not part of the electronic device (520) in the FIG. 5example) that may capture video image(s) to be coded by the videoencoder (503). In another example, the video source (501) is a part ofthe electronic device (520).

The video source (501) may provide the source video sequence to be codedby the video encoder (503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (550). In some embodiments, the controller(550) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (550) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (550) can be configured to have other suitablefunctions that pertain to the video encoder (503) optimized for acertain system design.

In some embodiments, the video encoder (503) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (530) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (533)embedded in the video encoder (503). The decoder (533) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (534). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (534) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4. Brieflyreferring also to FIG. 4, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415), andparser (420) may not be fully implemented in the local decoder (533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(535) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (535), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller (550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 6 shows a diagram of a video encoder (603) according to anotherembodiment of the disclosure. The video encoder (603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (603) is used in theplace of the video encoder (303) in the FIG. 3 example.

In an HEVC example, the video encoder (603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (603) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621), andan entropy encoder (625) coupled together as shown in FIG. 6.

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intramode, the general controller (621) controls the switch (626) to selectthe intra mode result for use by the residue calculator (623), andcontrols the entropy encoder (625) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(621) controls the switch (626) to select the inter prediction resultfor use by the residue calculator (623), and controls the entropyencoder (625) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (603) also includes a residuedecoder (628). The residue decoder (628) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (622) and theinter encoder (630). For example, the inter encoder (630) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (622) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (625) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7.

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (772) or the inter decoder (780), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (780); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (772). The residual information can be subject to inversequantization and is provided to the residue decoder (773).

The inter decoder (780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (773) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (771) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503), and (603), and thevideo decoders (310), (410), and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503),and (603), and the video decoders (310), (410), and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503), and (503), and the videodecoders (310), (410), and (710) can be implemented using one or moreprocessors that execute software instructions.

Aspects of the disclosure provide techniques for sub-block motion vectorclipping. The techniques are related to motion vector (MV) constraints,and can be used in advanced video codec to reduce memory bandwidthrequirement in affine mode.

In some embodiments, a motion vector may have an integer-pixel precisionsuch that the motion vector points to a pixel position for identifying areference block. In some embodiments, a motion vector may have afractional-pixel precision such that the motion vector points to afractional pixel position for identifying a reference block. Tocalculate pixel values at fractional pixel positions, interpolationfilters may be used, which may require additional pixels outside theintended reference block for interpolation operations.

For example, in HEVC, to calculate pixel values at fractional pixelpositions, 8-tap and 4-tap separable interpolation filters are used forluma and chroma components, respectively. For an M×N luma blockinterpolation, (M+7)×(N+7) luma samples need to be loaded from areference picture according to the integer-pixel parts of the motionvectors, M and N are positive integers. Accordingly, in HEVC, for a 4×4luma uni-directional inter prediction, the decoder may need to load upto (4+7)×(4+7)=121 luma samples for performing the interpolationprocess. The per-pixel memory bandwidth requirement for this example isabout 7.6 sample/pixel (121 samples for a 16-pixel block). Also, for a4×4 luma bi-directional inter prediction, the requirement may double tobecome 15.125 sample/pixel.

In some embodiments, limiting the size of a block that is coded usinginter prediction reduces the memory bandwidth requirement. For example,if the minimal block size of a block coded using the bi-directionalinter prediction is limited to 8×8 pixels, the per-pixel memorybandwidth requirement can be reduced to 7.0 sample/pixel (450 samplesfor a 64-pixel block). Therefore, in some embodiments for interprediction blocks smaller than 8×8, only uni-directional interprediction is allowed.

FIG. 8 is a schematic illustration of spatial neighboring blocks thatcan be used to determine motion information for a current block (801)using an affine motion compensation method in accordance with anembodiment. FIG. 8 shows a current block (801) and its spatialneighboring blocks denoted A0, A1, A2, B0, B1, B2, and B3 (802, 803,807, 804, 805, 806, and 808, respectively). In some examples, spatialneighboring blocks A0, A1, A2, B0, B1, B2, and B3 and the current block(801) belong to a same picture.

Affine motion compensation, by describing a 6-parameter (or a simplified4-parameter) model for a coding block, such as the current block (801),can efficiently predict the motion information for all samples withinthe current block with respect to a particular reference picture in aparticular prediction direction. In some embodiments, in an affine codedor described coding block, different part of the samples can havedifferent motion vectors with respect to the particular reference. Thebasic unit to have a motion vector in an affine coded or described blockis referred to as a sub-block. The size of a sub-block can be as smallas 1 sample only; and can be as large as the size of current block.

In some examples, an affine model uses 6 parameters to describe themotion information of an affine coded block, which can be represented bythree motion vectors (also referred to as three control point motionvectors) at three different locations of the block. In the FIG. 8example, CP0 is located at top-left corner of the block, CP1 is locatedat top-right corner of the block, and CP2 is located at bottom-leftcorner of the block.

In another example, a simplified affine model uses four parameters todescribe the motion information of an affine coded block, which can berepresented by two motion vectors (also referred to as two control pointmotion vectors) at two different locations of the block (e.g., controlpoints CP0 and CP1 at top-left and top-right corners in FIG. 8).

In some embodiments, the motion information for the control points CP0,CP1, and CP2 can be derived from motion information of the spatialneighboring blocks A0, A1, A2, B0, B1, B2, and B3. For example, thecontrol points CP0 can be derived based on checking the motioninformation of the spatial neighboring blocks B2, A2, and B3; thecontrol points CP1 can be derived based on checking the motioninformation of the spatial neighboring blocks B0 and B 1; and thecontrol points CP2 can be derived based on checking the motioninformation of the spatial neighboring blocks A0 and A1.

When an affine model is determined, the motion vector (with respect tothe particular reference picture) can be derived using such a model. Insome embodiments, in order to reduce implementation complexity, theaffine motion compensation is performed on a sub-block basis, instead ofon a sample basis. Accordingly, in such embodiments, each sub-blockwithin the current block has a corresponding motion vector with respectto the particular reference that is applicable to all samples in therespective sub-block. In some examples, the representative location ofeach sub-block can be signaled or predetermined according to a videocoding standard. In some examples, a location of a sub-block can berepresented by a top-left or a center point of the sub-block. In anexample using VVC, a sub-block may have a size of 4×4 samples.

In some embodiments, the affine prediction can be bi-directional affinethat performs bi-directional prediction on blocks of 4×4 samples. Thebi-direction affine prediction can increase memory bandwidth requirementa lot and is not preferred by hardware implementation in some examples.

In some examples of 6-parameter affine models, affine parameters a, b,c, d, e, f are used, a motion vector can be represented as (Eq. 1):

$\begin{matrix}\left\{ \begin{matrix}{{MVx} = {{ax} + {by} + e}} \\{{MVy} = {{cx} + {dy} + f}}\end{matrix} \right. & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$where MVx denotes the x-component of the motion vector and MVy denotesthe y-component of the motion vector.

FIG. 9 shows a diagram for illustrating a 6-parameter affine model. Inthe FIG. 9 example, the three control points CP0, CP1 and CP2 of theblock have corresponding CPMVs respectively associated with the controlpoints. For example, CPMV0 is associated with CP0, CPMV1 is associatedwith CP1, and CPMV2 is associated with CP2.

Each CPMV can be represented as a combination of a x-component and ay-component. For example, CPMV0 has x-component MVx0, and y-componentMVy0, CPMV1 has x-component MVx1, and y-component MVy1, CPMV2 hasx-component MVx2, and y-component MVy2.

In some embodiments, when the width of the block is W and height of theblock is H, and according to (Eq. 1), the three control points CPMV0,CPMV1 and CPMV2 can be represented in the terms of a, b, c, d, e, f, W,and H as:

CPMV0:(e, f)

CPMV1:(a·W+e, c·W+f)

CPMV2:(b·H+e, d·H+f)

Then, the affine parameters a, b, c, d may be derived according to (Eq.2):

$\begin{matrix}\left\{ \begin{matrix}{a = \frac{{{MVx}\; 1} - {{MVx}\; 0}}{W}} \\{b = \frac{{{MVx}\; 2} - {{MVx}\; 0}}{H}} \\{c = \frac{{{MVy}\; 1} - {{MVy}\; 0}}{W}} \\{d = \frac{{{MVy}\; 2} - {{MVy}\; 0}}{H}}\end{matrix} \right. & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

In some examples, a is considered as the change rate of the x-componentof the motion vector to x, b is considered as the change rate of thex-component to y, c is considered as the change rate of the y-componentof the motion vector to x, and d is considered as the change rate of they-component to y.

According to some aspects of the disclosure, sub-block motion vectorsare constrained (e.g., clipped) to reduce the memory bandwidth forsub-block based modes, including affine based mode and planar basedmode.

In an embodiment, a sub-block motion vector clipping technique is used.In some examples, for a W×H block (W denotes width, and H denotes heightfo the block) in inter prediction, when the block is coded in asub-block based mode, such as affine, ATMVP, planar MV prediction, theMVs of sub-blocks inside an M×N block area (M<=W, N<=H) are constrainedsuch that the maximum absolute difference between the integer part ofeach component of sub-block MVs of each prediction list is no largerthan a certain threshold, such as 0, 1, 2, 3 integer pixels. The M×Nblock area is referred to as a clipping region in some examples.

According to an aspect of the disclosure, a technique that is referredto as adaptive sub-block MV clipping technique is used. In someexamples, motion vectors of sub-blocks within the M×N block region(clipping region) are clipped adaptively based on a maximum motionvector value and a minimum motion vector value MV in the region.

Specifically, in some embodiments, for an M×N block area (M is the widthof the area and N is the height of the area) that is divided into Ssub-blocks (S is a positive integer), and an i-th motion vector of ani-th sub-block can be denoted as (MVxi, MVyi), where i is an integernumber from 1 to S, MVxi is the x-component and MVyi is the y-componentof the i-th motion vector. In some examples, MVminx denotes to minimumx-component of the S sub-blocks, for example, is calculated by min{MVxi}with i from 1 to S; MVmaxx denotes to maximum x-component of the Ssub-blocks, for example, is calculated by max{MVxi} with i from 1 to S;MVminy denotes to minimum y-component of the S sub-blocks, for example,is calculated by min{MVyi} with i from 1 to S; MVmaxy denotes to maximumy-component of the S sub-blocks, for example, is calculated by max{MVyi}with i from 1 to S.

Further, MV precision denotes the number of bits to represent theprecision of MV components. In an example, when MV precision is 4, theprecision is 1/16-pel; and when MV precision is 3, the precision is⅛-pel. Further, T[x] denotes the maximum allowed integer differencebetween x components of motion vectors of the target reference list inthe clipping process, and T[y] denotes the maximum integer differencebetween y components of the motion vectors of the target reference listin the clipping process. For example, when T[x] is 0, the motion vectorsof the sub-blocks share the same integer for x component; when T[y]=0,the motion vectors of the sub-blocks share the same integer for ycomponent. Further, Offset[x], and Offset[y] denote the maximumallowable differences of the x components and y components respectively.Then, Offset[x] and Offset[y] can be calculated byOffset[x]=(((1+T[x])<<MV precision)−1) and Offset[y]=(((1+T[x])<<MVprecision)−1).

Further, in some embodiments, the difference of motion vectors ofsub-blocks in a clipping region may be larger than the maximum allowabledifference, then the upper boundary (e.g., respectively for x componentand y component) and the lower boundary (e.g., respectively for xcomponent and y component) for clipping are determined. Then, when amotion vector direction component (e.g., x component or y component) ofa sub-block in the clipping region is higher than the upper boundary(e.g., x component or y component), the motion vector directioncomponent is clipped to the upper boundary; and when a motion vectordirection component (e.g., x component or y component) of a sub-block inthe clipping region is smaller than the lower boundary (e.g., xcomponent or y component), the motion vector direction component isclipped to the lower boundary.

The process to determine the lower boundary and the upper boundary isreferred to as adaptive sub-block MV clipping process. Because theprocess may be applicable to the x components and y components in asimilar approach, the following pseudo code example is based on the xcomponents, and can be suitably modified for the y-components:

MVminx = min{MVxi}; // get minimum MV component of sub-block MVs of thetarget reference list along a first coordinate direction (e.g., xcoordinate direction) MVmaxx = max{MVxi}; // get maximum MV component ofsub-block MVs of the target reference list along the first coordinatedirection (e.g., x coordinate direction) roundMVminx = MVminx >>MV_precision << MV_precision; // get integer part of MVminx roundMVmaxx= MVmaxx >> MV_precision << MV_precision; // get integer part of MVmaxxif ((MVminx − roundMVminx) < (MVmaxx − roundMVmaxx)) {// mainly usemin{MVxi} for the clipping process when min{MVxi} is closer to anroundMVminx MVminx = roundMVminx; MVmaxx = MVminx + Offset[x]; } else{// mainly use max{MVxi} for the clipping process when max{MVxi} iscloser to roundMVmaxx MVmaxx = roundMVmaxx; MVminx = MVmaxx − Offset[y];}

The above pseudo code example can be suitably modified for y componentsto determine MVmaxy and MVmaxy. Then, the motion vectors of thesub-blocks of the target reference list (e.g., L0 or L1) in the clippingregion can be suitably clipped to MVmaxx, MVmaxy, MVminx, MVminy.

According to the pseudo code example, the determining the range of thetarget motion vectors for the group of blocks along a particularcoordinate direction includes determining a maximum value of the basemotion vectors, along the first coordinate direction, for the group ofblocks, and determining a minimum value of the base motion vectors,along the first coordinate direction, for the group of blocks. Inresponse to a determination that a first difference between the minimumvalue and a first integer-pixel part of the minimum value is less than asecond difference between the maximum value and a second integer-pixelpart of the maximum value, the lower bound value of the range can bedetermined according to the minimum value of the base motion vectors,and the upper bound value of the range can be determined according to aninteger-pixel portion of the determined lower bound value, the targetdifference, and a precision setting of the target motion vectors. Also,in response to a determination that the first difference is not lessthan the second difference, the upper bound value of the range can bedetermined according to the maximum value of the base motion vectors,and the lower bound value of the range can be determined according to aninteger-pixel portion of the determined upper bound value, the targetdifference, and the precision setting of the target motion vectors.

According to an aspect of the disclosure, in the affine mode, sub-blockbased motion vectors are used for reconstruction. The clipping methodscan be further modified to improve coding efficiency. In followingexamples, a clipping region includes 8×8 samples. The examples can besuitably modified for other suitable sizes of clipping regions.

According to some aspects of the disclosure, due to clipping, affinemodels of different clipping regions in an affine coded block can bedifferent from an original affine model of the affine coded block. Thepresent disclosure provides techniques to ensure that the differences ofthe affine models of different clipping regions to the original affinemodel are relatively small, such as smaller than a threshold.

In some embodiments, after the motion vectors of the sub-blocks in thefirst 8×8 region are derived using the original affine model. Thefollowing methods may be used to ensure the affine model of each of 8×8regions is not much different from the original affine model asdescribed by the affine control point motion vectors, or described byaffine parameters directly.

In an embodiment, all 4×4 sub-blocks of the first 8×8 region have theirmotion vectors clipped using the adaptive sub-block MV clippingtechnique described above, and a modified affine model is derived basedon the clipped motion vectors.

In an embodiment, affine parameters a′, b′, c′, and d′ are derived fromthe clipped motion vectors of the first 8×8 block.

FIG. 10 shows a diagram illustrating motion vector clipping in a firstclipping region, such as the first 8×8 block (1010). In the FIG. 10example, each small square is a 4×4 sub-block for the current CU 1000.The first 8×8 block (1010) includes sub-blocks (1011)-(1014). The first8×8 block (1010) is also referred to as a first clipping region. In anexample, a first affine model (also referred to as an original affinemodel) is determined based on control points of the CU 1000, such asCP0, CP1 and CP2 shown in FIG. 10. In an example, the first affine modelhas the format shown in (Eq. 1), and has the change rate parameters a,b, c, d. Based on the first affine model, motion vectors of four 4×4sub-blocks (1011)-(1014) in the first 8×8 block (1010), such as shown byMV1-MV4 in FIG. 10, are derived. In an example, a motion vector at acenter of a 4×4 sub-block is calculated based on the first affine model,and is used as the motion vector for the sub-block.

Then, in some embodiments, the motion vectors MV1-MV4 are suitablyclipped according to the adaptive sub-block MV clipping technique, andthe clipped motion vectors can be used to derive a second affine model(also referred to as a clipped affine model). In an example, the secondaffine model is in the format of (Eq. 1), and has rate changeparameters, a′, b′, c′, d′.

FIG. 11 shows a diagram illustrating the first clipping region 1010. Inthe FIG. 11 example, the motion vector MV1 is clipped to MV1′, motionvector MV2 is clipped to MV2′, and motion vector MV3 is clipped to MV3′.The motion vector MV1′ at the center of the sub-block (1011) is used asa control point CP′0, the motion vector MV2′ at the center of thesub-block (1012) is used as a control point CP′1, The motion vector MV3′at the center of the sub-block (1013) is used as a control point CP′2.The control points CP′0-CP′2 are used to derive the second affine model,for example having affine parameters a′, b′, c′, and d′. The width andheight used for the calculation are both 4, as shown in FIG. 11.

Subsequently, the sub-block MVs of all remaining 8×8 regions in the CU(1000) may be derived by the second affine model having derived affineparameters (a′, b′, c′, d′). Then, the adaptive sub-block clippingtechnique is applied on sub-block MVs of the remaining 8×8 regions ofthis CU (1000).

In another embodiment, the first affine model (the original affinemodel) of the CU (1000) is used to derive motion vectors of allsub-blocks. Subsequently, adaptive sub-block MV clipping is applied foreach 8×8 region. After the clipping, the affine parameters (a′, b′, c′,d′) are derived from each 8×8 region. Further, affine parameters a′, b′,c′, d′ are compared with the original affine parameters a, b, c and d.When the affine parameters a′, b′, c′, d′ are different from theoriginal affine parameters a, b, c and d, and the differences are abovea predefined threshold, the new parameters a′, b′, c′, d′ are clippedfor the region and then sub-block MVs for the corresponding region arere-derived from the clipped affine parameters. In some examples, there-derived sub-block MVs are further clipped according to the adaptivesub-block MV clipping technique.

In another embodiment, affine parameters a, b, c, d are first derivedfor an affine block based on CPMVs of the block. Then, the affineparameters a, b, c, d are clipped to certain ranges, such as |a|<=T[1],|b|<=T[2], |c|<=T[3], |d|<=T[4], where T[1], T[2], T[3] and T[4] arerange parameters for a, b, c, d respectively.

In some examples, the clipped affine parameters are used to derivesub-block MVs at clipping region level, such as 8×8 level.

In another embodiment, the clipped affine parameters are used to derivesub-block MVs at 8×8 (M×N) level. Further, the newly derived sub-blockMVs are clipped using adaptive sub-block MV clipping technique.

It is noted that T[1], T[2], T[3] and T[4] can be the same or differentfor affine parameters, a, b, c, d.

In an example, T[1], T[2], T[3] and T[4] are 2 for all affineparameters. In another example, T[1], T[2], T[3] and T[4] are 8 for allaffine parameters.

In some examples, T[1], T[2], T[3] and T[4] may be predefined at bothencoder and decoder. In another embodiment, one or more of T[1], T[2],T[3] and T[4] is/are signaled in the bitstream, such as in sequenceparameter set (SPS), picture parameter set (PPS), slice header, tileheader, video usage information (VUI), or supplemental enhancementinformation (SEI) message.

In some examples, the proposed clip is applied to all affine blocks. Inanother embodiment, the proposed clip is only applied bi-predicted (ormulti-hypothesis) affine blocks.

FIG. 12 shows a flow chart outlining a process (1200) according to anembodiment of the disclosure. The process (1200) can be used in thereconstruction of a block coded in intra mode, so to generate aprediction block for the block under reconstruction. In variousembodiments, the process (1200) are executed by processing circuitry,such as the processing circuitry in the terminal devices (210), (220),(230) and (240), the processing circuitry that performs functions of thevideo encoder (303), the processing circuitry that performs functions ofthe video decoder (310), the processing circuitry that performsfunctions of the video decoder (410), the processing circuitry thatperforms functions of the video encoder (503), and the like. In someembodiments, the process (1200) is implemented in software instructions,thus when the processing circuitry executes the software instructions,the processing circuitry performs the process (1200). The process startsat (S1201) and proceeds to (S1210).

At (S1210), prediction information of a current block in a currentpicture is decoded from a coded video bitstream. The predictioninformation is indicative of an affine based motion vector prediction.

At (S1220), a first affine model for the current block is determined.The first affine model has first affine parameters. In an example, thefirst affine model is determined based on control points of the currentblock. In some embodiments, the first affine model is represented in theformat of (Eq. 1), and the affine parameters are a, b, c, d, e, f. Then,a is the change rate of the x-component of the MV to x, b is the changerate of the x-component to y, c is the change rate of the y-component ofthe MV to x, d is the change rate of the y-component to y, and e and fare the x-component and y-component of one of the CPMVs.

At (S1230), a second affine model for a clipping region is determined.The second affine model has second affine parameters. The differencesbetween the second affine parameters and the corresponding firstparameters are below a threshold. Motion vectors determined based on thesecond affine model satisfy a clipping requirement. In some examples,the second affine model has second affine parameters a′, b′, c′, d′. Theabsolute difference between a and a′, the absolute difference between band b′, the absolute difference between c and c′, and the absolutedifference between d and d′ are smaller than a threshold. Thus, thesecond affine model is very close to the first affine model. Motionvectors of sub-blocks in the clipping region satisfy a clippingrequirement, such as having the same integer part, difference of integerparts being smaller than a threshold, and the like.

At (S1240), samples of the clipping region are reconstructed accordingto the second affine model. In an example, a reference sample in thereference picture that corresponds to a sample in the clipping region isdetermined according to the second affine model. Further, the sample inthe clipping region is reconstructed according to the reference samplein the reference picture. In some embodiments, when the motion vectorsin the clipping region satisfy the clipping requirement, the number ofreference pixels to load, from a main memory to an on-chip memory forexample, can be reduced, and the memory bandwidth requirement can bereduced. Then, the process proceeds to (S1299), and terminates.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 13 shows a computersystem (1300) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 13 for computer system (1300) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1300).

Computer system (1300) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1301), mouse (1302), trackpad (1303), touchscreen (1310), data-glove (not shown), joystick (1305), microphone(1306), scanner (1307), camera (1308).

Computer system (1300) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1310), data-glove (not shown), or joystick (1305), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1309), headphones(not depicted)), visual output devices (such as screens (1310) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1300) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1320) with CD/DVD or the like media (1321), thumb-drive (1322),removable hard drive or solid state drive (1323), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1300) can also include an interface to one or morecommunication networks. Networks can for example be wireless, wireline,optical. Networks can further be local, wide-area, metropolitan,vehicular and industrial, real-time, delay-tolerant, and so on. Examplesof networks include local area networks such as Ethernet, wireless LANs,cellular networks to include GSM, 3G, 4G, 5G, LTE and the like, TVwireline or wireless wide area digital networks to include cable TV,satellite TV, and terrestrial broadcast TV, vehicular and industrial toinclude CANBus, and so forth. Certain networks commonly require externalnetwork interface adapters that attached to certain general purpose dataports or peripheral buses (1349) (such as, for example USB ports of thecomputer system (1300)); others are commonly integrated into the core ofthe computer system (1300) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1300) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1340) of thecomputer system (1300).

The core (1340) can include one or more Central Processing Units (CPU)(1341), Graphics Processing Units (GPU) (1342), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1343), hardware accelerators for certain tasks (1344), and so forth.These devices, along with Read-only memory (ROM) (1345), Random-accessmemory (1346), internal mass storage such as internal non-useraccessible hard drives, SSDs, and the like (1347), may be connectedthrough a system bus (1348). In some computer systems, the system bus(1348) can be accessible in the form of one or more physical plugs toenable extensions by additional CPUs, GPU, and the like. The peripheraldevices can be attached either directly to the core's system bus (1348),or through a peripheral bus (1349). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1341), GPUs (1342), FPGAs (1343), and accelerators (1344) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1345) or RAM (1346). Transitional data can be also be stored in RAM(1346), whereas permanent data can be stored for example, in theinternal mass storage (1347). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1341), GPU (1342), massstorage (1347), ROM (1345), RAM (1346), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1300), and specifically the core (1340) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1340) that are of non-transitorynature, such as core-internal mass storage (1347) or ROM (1345). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1340). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1340) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1346) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1344)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

APPENDIX A: ACRONYMS

JEM: joint exploration model

VVC: versatile video coding

BMS: benchmark set

MV: Motion Vector

HEVC: High Efficiency Video Coding

SEI: Supplementary Enhancement Information

VUI: Video Usability Information

GOPs: Groups of Pictures

TUs: Transform Units,

PUs: Prediction Units

CTUs: Coding Tree Units

CTBs: Coding Tree Blocks

PBs: Prediction Blocks

HRD: Hypothetical Reference Decoder

SNR: Signal Noise Ratio

CPUs: Central Processing Units

GPUs: Graphics Processing Units

CRT: Cathode Ray Tube

LCD: Liquid-Crystal Display

OLED: Organic Light-Emitting Diode

CD: Compact Disc

DVD: Digital Video Disc

ROM: Read-Only Memory

RAM: Random Access Memory

ASIC: Application-Specific Integrated Circuit

PLD: Programmable Logic Device

LAN: Local Area Network

GSM: Global System for Mobile communications

LTE: Long-Term Evolution

CANBus: Controller Area Network Bus

USB: Universal Serial Bus

PCI: Peripheral Component Interconnect

FPGA: Field Programmable Gate Areas

SSD: solid-state drive

IC: Integrated Circuit

CU: Coding Unit

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method for video decoding in a decoder,comprising: decoding prediction information of a current block in acurrent picture from a coded video bitstream, the prediction informationbeing indicative of an affine based motion vector prediction;determining a first affine model for the current block, the first affinemodel having first affine parameters; determining first motion vectorsfor sub-blocks in each clipping region of the current block based on thefirst affine model; clipping the first motion vectors for each clippingregion of the current block to generate second motion vectors forrespective clipping regions that satisfy a clipping requirement;determining respective second affine models for the clipping regionsbased on the second motion vectors for the respective clipping regions;constraining second affine parameters of the second affine models withdifferences to the first affine parameters being smaller than athreshold; and reconstructing samples in the clipping regions based onthe respective second affine models for the clipping regions, whereinthe determining the respective second affine models includes determininga second affine model for a first clipping region in the current block,motion vectors determined based on the second affine model satisfyingthe clipping requirement; and the reconstructing includes reconstructinga sample in the first clipping region based on the second affine model.2. The method of claim 1, wherein the determining the first motionvectors includes determining the first motion vectors for the sub-blocksin the first clipping region based on the first affine model; theclipping the first motion vectors includes clipping the first motionvectors for the sub-blocks in the first clipping region to generate thesecond motion vectors for the first clipping region that satisfy theclipping requirement; and the determining the respective second affinemodels includes determining the second affine model for the firstclipping region based on the second motion vectors of the sub-blocks inthe first clipping region.
 3. The method of claim 2, wherein thereconstructing comprises: reconstructing a sample in a second clippingregion within the current block based on the second affine model for thefirst clipping region.
 4. The method of claim 1, wherein the clippingcomprises: clipping the first affine parameters based on a rangerequirement to generate the second affine parameters for the secondaffine model for the first clipping region.
 5. The method of claim 4,further comprising: determining motion vectors of sub-blocks for eachclipping region based on the second affine model for the first clippingregion; and clipping the motion vectors of the sub-blocks.
 6. The methodof claim 4, wherein the range requirement is predefined.
 7. The methodof claim 4, further comprising: decoding a signal from the coded videobitstream that is indicative of the range requirement.
 8. The method ofclaim 1, wherein the current block is a bi-predicted block.
 9. Anapparatus for video decoding, comprising: processing circuitryconfigured to: decode prediction information of a current block in acurrent picture from a coded video bitstream, the prediction informationbeing indicative of an affine based motion vector prediction; determinea first affine model for the current block, the first affine modelhaving first affine parameters; determine first motion vectors forsub-blocks in each clipping region of the current block based on thefirst affine model; clip the first motion vectors for each clippingregion of the current block to generate second motion vectors forrespective clipping regions that satisfy a clipping requirement;determine respective second affine models for the clipping regions basedon the second motion vectors for the respective clipping regions;constrain second affine parameters of the second affine models withdifferences to the first affine parameters being smaller than athreshold; and reconstruct samples in the clipping regions based on therespective second affine models for the clipping regions, wherein theprocessing circuitry is configured to determine a second affine modelfor a first clipping region in the current block, motion vectorsdetermined based on the second affine model satisfying the clippingrequirement; and reconstruct a sample in the first clipping region basedon the second affine model.
 10. The apparatus of claim 9, wherein theprocessing circuitry is configured to: determine the first motionvectors for the sub-blocks in the first clipping region based on thefirst affine model; clip the first motion vectors for the sub-blocks inthe first clipping region to generate the second motion vectors for thefirst clipping region that satisfy the clipping requirement; anddetermine the second affine model for the first clipping region based onthe second motion vectors of the sub-blocks in the first clippingregion.
 11. The apparatus of claim 10, wherein the processing circuitryis configured to: reconstruct a sample in a second clipping regionwithin the current block based on the second affine model for the firstclipping region.
 12. The apparatus of claim 9, wherein the processingcircuitry is configured to: clip the first affine parameters based on arange requirement to generate the second affine parameters for thesecond affine model for the first clipping region.
 13. The apparatus ofclaim 12, wherein the processing circuitry is configured to: determinemotion vectors of sub-blocks for each clipping region based on thesecond affine model for the first clipping region; and clip the motionvectors of the sub-blocks.
 14. The apparatus of claim 12, wherein therange requirement is predefined.
 15. The apparatus of claim 12, whereinthe processing circuitry is configured to: decode a signal from thecoded video bitstream that is indicative of the range requirement. 16.The apparatus of claim 9, wherein the current block is a bi-predictedblock.
 17. A non-transitory computer-readable medium storinginstructions which when executed by a computer for video decoding causethe computer to perform: decoding prediction information of a currentblock in a current picture from a coded video bitstream, the predictioninformation being indicative of an affine based motion vectorprediction; determining a first affine model for the current block, thefirst affine model having first affine parameters; determining firstmotion vectors for sub-blocks in each clipping region of the currentblock based on the first affine model; clipping the first motion vectorsfor each clipping region of the current block to generate second motionvectors for respective clipping regions that satisfy a clippingrequirement; determining respective second affine models for theclipping regions based on the second motion vectors for the respectiveclipping regions; constraining second affine parameters of the secondaffine models with differences to the first affine parameters beingsmaller than a threshold; and reconstructing samples in the clippingregions based on the respective second affine models for the clippingregions, wherein the determining the respective second affine modelsincludes determining a second affine model for a first clipping regionin the current block, motion vectors determined based on the secondaffine model satisfying the clipping requirement; and the reconstructingincludes reconstructing a sample in the first clipping region based onthe second affine model.
 18. The non-transitory computer-readable mediumof claim 17, wherein the instructions cause the computer to perform:determining the first motion vectors for the sub-blocks in the firstclipping region based on the first affine model; clipping the firstmotion vectors for the sub-blocks in the first clipping region togenerate the second motion vectors for the first clipping region thatsatisfy the clipping requirement; and determining the second affinemodel for the first clipping region based on the second motion vectorsof the sub-blocks in the first clipping region.